SNVA994A February   2022  – March 2023 LM5157 , LM5157-Q1 , LM51571-Q1 , LM5158 , LM5158-Q1 , LM51581 , LM51581-Q1

 

  1.   1
  2.   Trademarks
  3. 1Introduction
  4. 2Example Application
  5. 3Calculations and Component Selection
    1. 3.1 Switching Frequency
    2. 3.2 Transformer Selection
      1. 3.2.1 Maximum Duty Cycle and Turns Ratio Selection
      2. 3.2.2 Primary Winding Inductance Selection
    3. 3.3 Slope Compensation Check
    4. 3.4 Diode Selection
    5. 3.5 Output Capacitor Selection
    6. 3.6 Input Capacitor Selection
    7. 3.7 UVLO Resistor Selection
    8. 3.8 Control Loop Compensation
      1. 3.8.1 Crossover Frequency (fcross) Selection
      2. 3.8.2 RCOMP Selection
      3. 3.8.3 CCOMP Selection
      4. 3.8.4 CHF Selection
  6. 4Component Selection Summary
    1. 4.1 Application Circuit
    2. 4.2 Bill of Materials
  7. 5Small Signal Frequency Analysis
    1. 5.1 Flyback Regulator Modulator Modeling
    2. 5.2 Compensation Modeling
  8. 6Revision History

3.8.4 CHF Selection

The CHF capacitor sets the high frequency pole of the compensation network. The high frequency pole aids in attenuating high frequency noise due to the switching frequency and assuring enough gain margin achieved. It is recommended to set the pole frequency between or between the RHP zero (ωZ_RHP), which is usually smaller than the ESR zero, or between the RHP zero and half the switching frequency. In this design example, the high frequency pole is placed at the RHP zero. Equation 25 is used to calculate the value of CHF.

Equation 25. CHF= DVIN_min × LM × NS12NP2 × POUT_totalVLOAD12RCOMP × (1- DVIN_min)2=  0.51 × 8µH × 1.2212 × 8.5W10V210kΩ × (1- 0.51)2=208pF

For the EVM the CHF is chosen to be 1 nF.